1. Field of the Invention
This invention relates to data transmission systems, and more particularly to a bifrequency encoded binary data transmission system wherein the transmitted signal is predistorted to eliminate the need for equalization at the receiving end.
2. Description of the Prior Art
The distance over which an information carrying signal can be transmitted by a transmission line is limited primarily by two factors: (1) the signal to noise ratio of the transmission, and (2) the phase distortion introduced by the transmission line. Increasing the signal to noise ratio of the transmission is relatively easily accomplished. On the other hand, transmission line induced phase distortion is a more difficult problem since, even with a good signal to noise ratio, detection of the information may be possible only after complex transformation of the received signal. In a bifrequency encoded binary data transmission system, an oscillograph of the transmitted signal reveals a characteristic pattern of two repeating small "eyes" each of which represents a half-bit period. The receiver gates on the center of these eyes for detection of the signal and on the level transitions for synchronization. Phase distortion which is a function of increasing line length has the effect of closing or causing the disappearance of these eyes with the result that detection of the signal is impossible.
In order to overcome the effects of phase distortion over long lines, it is known to provide an equalizer or filter at the end of the transmission line. The purpose of the equalizer is to introduce an inverse distortion to compensate for the transmission line distortion. But, since the line distortion is a function of line length or distance, the equalizer must be tuned to the distance. This means that any change in line length effectively requires a redesign of the equalizer or an automatic equalizer. In addition to the inflexibility of equalizers, they have the additional disadvantage of being expensive to fabricate.